Thermal coking processes allow crude oil refineries to process heavier hydrocarbons present in petroleum, tar sands, and other hydrocarbon sources. In general, thermal coking processes employ high-severity, thermal decomposition (or “cracking”) to maximize the conversion of very heavy, low-value residuum feeds to lower boiling hydrocarbon products of higher value. Feedstocks for these coking processes normally consist of refinery process streams which cannot economically be further distilled, catalytically cracked, or otherwise processed to make fuel-grade blend streams. Typically, these materials are not suitable for catalytic operations because of catalyst fouling and/or deactivation by ash and metals. Common coking feedstocks include atmospheric distillation residuum, vacuum distillation residuum, catalytic cracker residual oils, hydrocracker residual oils, and residual oils from other refinery units.
Three types of coking processes used in crude oil refineries and upgrading facilities to convert the heavy hydrocarbon fractions into lighter hydrocarbons and petroleum coke include delayed coking, fluid coking, and flexicoking. In all three of these coking processes, the petroleum coke is considered a by-product that is tolerated in the interest of more complete conversion of refinery residues to lighter hydrocarbon compounds. The resulting hydrocarbons and other products move from the coking vessel to a fractionator in vapor form. The heavier cracked liquids (e.g. gas oils) are commonly used as feedstocks for further refinery processing (e.g. Fluid Catalytic Cracking Units or FCCUs) that transforms them into transportation fuel blend stocks.
Crude oil refineries have regularly increased the use of heavier crudes in their crude blends due to greater availability and lower costs. These heavier crudes have a greater proportion of the heavier hydrocarbon components, increasing the need for coker capacity. Thus, the coker often becomes a bottleneck that limits refinery throughput. Also, these heavier crudes often contain higher concentrations of large, aromatic structures (e.g. asphaltenes and resins) that contain greater concentrations of sulfur, nitrogen, and heavy metals, such as vanadium and nickel.
As a result, the coking reactions (or mechanisms) are substantially different and tend to produce a denser, shot (vs. sponge) coke crystalline structure (or morphology) with higher concentrations of undesirable contaminants in the pet coke and coker gas oils. Unfortunately, many of the technology improvements attempting to deal with the above (plant capacity/bottlenecks, feedstock compositional changes, etc.) have substantially decreased the quality of the resulting pet coke. Most of the technology improvements and heavier, sour crudes tend to push the pet coke from porous sponge coke to shot coke with higher concentrations of undesirable impurities. The resulting shift in coke quality can require a major change in coke markets (e.g. anode to fuel grade) and dramatically decrease coke value. The changes in technology and associated feed changes can result in decreased quality of the fuel grade coke, having lower volatile matter and gross heating value, among other properties, making the produced fuel grade coke less desirable.